(In [74571]) Just forward Stricmp() in wx/string.h to wxCRT_StricmpA().

Don't redo the tests already done in wx/wxcrtbase.h in wx/string.h too,
especially as they were not done correctly there (they didn't take into
account the case of MinGW in strict ANSI mode). Just call wxCRT_StricmpA().

This also allows us to get rid of HAVE_STRCASECMP_IN_STRING[S]_H tests in
configure.